Two Stroke And Four Stroke Engine

To understand the differences between a two stroke and four stroke engines, we need to know how the four stroke engine works. In four stroke engine there are four stages:

Intake: The piston travels down the cylinder while the intake valve is opened to allow a mixture of fuel and air to enter the combustion chamber.

Compression: The intake valve is closed and the piston travels back up the cylinder compressing the gasses.

Combustion: The spark plug ignites the compressed gas causing it to explode, which forces the piston down.

Exhaust: The piston rises up the cylinder as the exhaust valve is opened, allowing the piston to clear the chamber to start the process over.

Each time the piston rises and falls it turns the crankshaft that is responsible for turning the wheels. This is how fuel is converted into forward motion.

The spark plug doesn’t fire in each revolution but instead fires once every other revolution. A camshaft must alternately tip a rocker arm attached either to the intake or exhaust valve. The rocker arm returns to its closed position according to the movement of a spring. The valves must be seated properly in the cylinder head to avoid compression leaks.

Now that the four stroke engine’s mechanism is explained, it is inevitable to learn about the two stroke engine. The main difference between the two engines is that all four stages in the four stroke engine are integrated into only two phases, one downward stroke, and one upward stroke. Intake and exhaust are both integrated into the compression and combustion movement of the piston, eliminating the need for valves. This is accomplished by an inlet and exhaust port in the wall of the combustion chamber itself.

Induction: the piston travels down the cylinder head, and it applies suction to the air-fuel mixture and exerts it into the cylinder.

Compression: as the piston travels up to the top of the cylinder head, it applies pressure to the air-fuel mixture from the inlet port in the top of the cylinder head, making the air-fuel mixture ready for igniting by the spark plug.

Ignition: the spark plug ignites the pressurized air-fuel mixture (otherwise known as the compressed air-fuel mixture), sometimes referred to as the power stroke.

Exhaust: as the piston returns back to the top of the cylinder head after the air-fuel mixture has been ignited, the piston pushes the burnt gases out of the cylinder and through the exhaust system.

Transfer Port: The port in a two stroke engine that transfers the air-fuel mixture from the bottom of the engine to the top of the cylinder.

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Fig2: 2-Stroke engine

After the fuel and air experience a relatively small explosion, the piston is driven down to the bottom of the stroke where the exhaust port is uncovered. Most of the gases are driven out in this phase. This process is easily seen with an outboard motorboat, evident by the multicoloured oil slick surrounding the engine, but it happens with all two stroke engines, which is considered one of the disadvantages that makes a designer overlook the use of a two stroke engine in their design. And that is because this process itself (along with burning oil) creates pollution and fuel efficiency issues.

Development

The idea to build a two-stroke engine goes back to the year 1879. But this engine became a qualitatively good product only after many years, when the German DKW company accelerated its development. Because of its disadvantages compared with the four-stroke engine, the two-stroke engine is used practically just in a small range of capacity, e.g. in small motorcycles considering that the processes involved in the two stroke engine only require the use of a small amount of parts and chambers, and that is why they are used in small applications. Formerly the engine was even used to power tiny cars.

The main disadvantage of the two stroke engine would be the loss of fuel and oil (as oil is mixed with fuel, or induced with fresh air to lubricate the piston). And this oil is burnt and sent through the exhaust, as some of the fresh mixture would leave the combustion chamber unburnt. This would basically cause more emissions and dirtier exhaust that would damage the catalytic converter.

In recent years, the re-appearance of direct fuel injection (as it already existed in diesel, and 1960’s petrol Mercedes), is the main cause of this interest. As direct fuel injection (meaning the fuel injector would be in the combustion chamber such as the spark plug, and would induce the fuel directly in there, as opposed to standard injection where fuel is induced in the intake manifold on top of the valve). This direct injection would limit the fresh mixture loss in exhaust.

The thermal post combustion: there is an air pump, called secondary air pump, pumping fresh air into the exhaust when engine is cold. As on cold starts you have a very rich mixture, meaning raw fuel leaving through the exhaust. The fresh air induced, along with the high temperatures of the exhaust, would cause the fuel in exhaust to burn, this would render the emissions cleaner, and would heat up the exhaust faster for the catalytic converter and oxygen sensors to work (as catalytic converters and oxygen sensors need temperatures around 600 degrees C to operate and clean the emissions).

This thermal post combustion would clean the remaining raw fuel or hydrocarbons, and heat the catalytic converters and oxygen sensors, so they always work at optimal temperatures.

Problems of the two-stroke engine

Actually, the two stroke engine should perform twice the performance of a four stroke engine with the same cubic capacity. Although it is just possible to gain a performance that is about 50% better, but still the reasons are obvious; the cylinder cannot be filled up with the same amount of fuel as in the four-stroke engine, because the individual strokes are separated not so clearly which will reduce the amount of space and eventually reduce the capacity for carrying more fuel. If more fuel is induced, it leaves the combustion chamber through the ejection pipe without being burnt. Many concepts were developed to provide a better expulsion of the exhaust in way that the fresh gas doesn’t leave the combustion chamber. Though all these inventions, the filling of the two-stroke engine is always worse than in the four-stroke engine, which loses fresh fuel only because of the “overlap” of the valve times (both valves are open for an instant). Beside these performance-technical problems, there are also increasing difficulties with the environment. The fuel mixture of the two-stroke engine often gets shifted with a certain quantity of oil because of the necessary lubrication. Unfortunately the oil gets burnt partly, too, and harmful gases are expulsed by the engine.

Two stroke engines already have a lot of advantages over the standard four stroke engines

Two-stroke engines do not have valves, which simplifies their construction and lowers their weight.

Two-stroke engines fire once every revolution, while four-stroke engines fire once every other revolution. This gives two-stroke engines a significant power boost.

– Two-stroke engines are lighter, and cost less to manufacture.

– Two-stroke engines have the potential for about twice the power in the same size because there are twice as many power strokes per revolution.

Disadvantages of 2 Stroke Engines:

-Two-stroke engines don’t live as long as four-stroke engines. The lack of a dedicated lubrication system means that the parts of a two-stroke engine wear-out faster. Two-stroke engines require a mix of oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls.

– Two-stroke oil can be expensive. Mixing ratio is about 4 ounces per gallon of gas: burning about a gallon of oil every 1,000 miles.

– Two-stroke engines do not use fuel efficiently, yielding fewer miles per gallon.

– Two-stroke engines produce more pollution from:

1- The combustion of the oil in the gas. The oil makes all two-stroke engines smoky to some extent, and a badly worn two-stroke engine can emit more oily smoke.

2- Each time a new mix of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port.

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